Recovery of zinc by electrolysis



n. AVERY AND N. H. STEVENS. l I

RECOVERY 0F ZINC BY ELECTROLYSIS.

APPLICATION FILED MAN. I6, 1920.

- tions it is customary to Iuse anodes of lead menaces.

fr raies,

DAVID AVERY, 0F MELBOURNE,

HOBART, TASMANIA, AUSTRALIA,

VICTORIA, 'AND ROYALE HILL'MIAN STEVENS, OF

ASSIGNORS T0 ELECTROLYTIC ZINC COMPANY 0F AUSTRALASIA. PROPRIETARY LIMITED, OF MELBOURNE, AUSTRALIA.

RECOVERY 0F ZINC BY ELECTROLYSIS.

To will witom z't may concern:

Beit known that we, DAVID AVERY, subject of the King of Great Britain, residing at Collins House, 360-366 Collins Street, Melbourne, in the State` of Victoria, Commonwealth of Australia, by occupation chemical, engineer, STEVENS, a citizen of the United States of America, residing at care of Electrolytic Zinc Company of Australasia Proprietary Limited, Risdon, Hobart, 'in the State of Tasmania, Commonwealth of Australia, by occupation metallurgist, have invented new and. useful Improvements in the Recovery of Zinc by Electrolysis, of which the following is a specification.

This invention relates to improvements in the recovery of zinc by electrolysis and refers more especially t0 the treatment of zinc solutions obtained when the roasted ores or calcines are leached with sulphuric acid (or spent electrolyte) and then subjected to electrolysis for the electro-deposition of zlnc therefrom.

In the electrolysis'of zinc bearing soluand cathodes of aluminumfand it has been found that disintegration of corrosion' of both these electrodes takes place, requiring frequent renewals and occasionlng consid-v erable losses. We have found that this corrosion is chiefly due to the presence of chlorine in the electrolyte which has an injurious effect on both the anodes and cathodes, and that if the chlorine is removed from the solution the amount of corrosion is eliminated or largely reduced and the life of the electrodes is eatly prolonged.

The presence of c lorine may be due to chlorides associated with the ores naturally or introduced during the .handling or treatment of the ores. i

The object of this invention is to provlde certain improvements in the electrolytic treatment of zinc bearing solutions whereby the detrimental effect of chlorine is avoided, more especially in preventing or reducing the corrosion. of the lead anodes and the aluminum cathodes in the cells used in such treatment.

We accomplish this object by removing the chlorine from the zinc bearing solution as insoluble silver chloride which is recovered subsequently reconverting the silver Specification of Letters Patent.

and ROYALE HILLMAN being carried on vention to the lPatenteol J an. IO, i922.

Application le'd March 16, 1920. Serial No. 366,361.

chloride into a s oluble silver salt whereby' the latter is agaln available as a chlorine precipitant.` In the electrolysis of zinc bearing solutions it is not always practicable to eliminate the whole of the chlorine from the mill circuit and we have found that a small amount of chlorine in the electrolyte may in some cases be permitted without serious effects, for example, when the electrolyte contains glue in solution and when the lead anodes are coated with a manganese dioxide deposit or scale, an amount of chlorine up to 50 milligrams per litre may be present without serious corrosion of electrodes occurring;A but when electrolysis is without the addition of glue or similar substances and/or when the anodes are not coated with a depositl of .manganese dioxide corrosion takes place more readily and the chlorine in the electrolyte should be eliminated as completely as possible.

The single ligure of the accompanying drawings illustrates by means of a conventional low sheet the novel features of the invention. This flow-sheet will be clearly understood from the following description.

ln carrying this invention into e'ect one methodA of ,preventing the accumulation of chlorine in the electrolyte beyond a certain predetermined point is by withdrawing a certain quantity of solution from'ithe circuit from time vto timeremoving the chlorine from this portion of the solution by treatment. with a soluble salt of'silver and returning the puried solution to the circuit. rIhe silver chloride formed is collected, re-

duced to metallic silver and reconverted into a soluble salt for re-use in subsequent operations. The quantity of solution which it will be necessary to treat for the removal of chlorine during any given period, can be determinedby consideration of the amount of chlorine which is present inthe solution at the beginning of the period, the amount of chlorine which would be introduced during the period and the amount of chlorine to which it is desired to reduce to the total contents of the electrolyte.

e prefer to effect the removal of the chlorine after the purification of the-solution with zinc dust. In applying this inrocedure in zinc electrolysis the predetermlned quantity of solution is diverted from the mill circuit preferably in convenient charges and introduced into asuitable vat in which silver chloride is to be precipitated. A sample of the solution .is taken and titrated with acid, the basicity of the solution determined and calculated in terms of sulphuric acid. The required quantity `of sulphuric acid is then added to the charge to render it slightly acid. Finely powdered silver sulphate is then added to the solution in quantity sufficient to remove the bulk of the chlorine present preferably leaving however 2 or 3 milligrams of chlorine per'litre in solution.

The mixture is now agitated until thereaction is complete which will vbe in about one hour, mill solution is then added until the charge is rendered neutral or slightly basic whereby the silver chloride is coagulated a result which is hastened by continuing the agitation. The precipitate is now allowed to settle and the supernatent liquor is filtered through a chamber type filter press (preferably wooden) the clear solution is then treated with a small amount of zine dust and again filtered to remove any traces of silver that remain in solution thereby obtaining ay lated in this way, it is mixed, after decanting the clear solution from the last charge, with the remaining bottom solution and pumped into a small tank to which is also added the cake ofsilver chloride obtained in the wooden filter press. To this sulfphuric acid and zinc dust are added and the whole agitated to reduce the silver chloride to metallic silver. The quantity of zinc dust added .should be sufficient to reduce the whole of the silver present which isassumed to'be equal to the amount of silver used in the form of sulphate during the process of chlorine precipitation, whereas the amount of sulphuric acid added should be sufficient to leave the solution slightly acid after the action is complete. It is necessary to have this slight excess lof sulphuric acid present otherwise zinc dustwould probably `remain with and contaminate the lreduced silver, on the other hand, care should be taken not to allow the solution to become too acid. When; the reaction is thought to `be complete'the silver precipitateuis tested for unreduced chloride by filtering ofi` a little of the precipitate, treating it on the filter with ammonium hydroxide, collecting the filtrate and acidifying it with .nitric acid. A white precipitate is formed if unreduced silver chloride is present. When all the silver chloride is reduced the precipitate of metallic silver is agitated for some time, care being taken to keep the solution slightly acid throughout, to ensure the remoyal of any.

excess zincl dust. The tank or vat is then discharged into a flat bottomed filter, the solution removed by the aid of a vacuum pump and the precipitate finally washed with water until free from chlorides.

.The silver is then removed dried weighed and introduced into a converting pan where pure sulphuric acid is added in calculated amount to convert the whole of the silver into the sulphate. The mixture is heated until the temperature reaches 250 C. or 300o C. and `is stirred, occasionally at first but more constantly towards the end of the operation so as to obtain the silver sulphate in the form of a granular mass. This is removed and ground to a fine powder in a Ball mill using flint pebbles. for reuse as a chlorine precipitant in sub- -se uent purification operations.

e claim `1. In the recovery of zinc by electrolysis from a zinc bearing solution containing chlorine, the step of removing chlorine to the desired extent from the zinc-bearing solution prior to electrolysis forthe purpose of reducing the corrosion of the electrodes durin the electrolysis of the solution.

2. li the recovery of zinc by electrolysis froml a zinc-bearing solution containing chlorine, the steps o removing chlorine to the desired extent from the zinc bearing solution prior to electrolysis for the purpose of reducing the corrosion of the electrodes during the electrolysis, and electrolyzing the resulting solution with anodes of lead and cathodes of aluminium.

3. In the recovery of zinc by electrolysis from a rzinc-bearing solution containing chlorine, the step of treating the zinc-bearing solution with a silver compound and thereby removing chlorine to the desired extent from the solution prior to electrolysis for the purpose of reducing the corrosion of the electrodes during the electrolysis.

4. In the recovery of zinc by electrolysis from a zinc-bearing solution containing chlorine, the ste of treating the zinc-bearing solution wit silver sulfate and thereby removing chlorine to the desired extent from the solution prior to electrolysis for the 'purpose of reducing the corrosionof` the electrodes during the electrolysis.

5. In the 'recoveryof zinc by electrolysis from a zinc-bearing solution containing chlorine, the steps of treating the zinc-bear- It is then ready ing solution with a silver compound and thereby removing chlorine by precipitation to the desired extent from the solution prior to electrolysis forthe purpose of reducing the corrosion of the electrodes during the electrolysis, and converting the precipitated silver chloride removed from the solution into metallic silver.

6. In the recovery of zinc' by electrolysis from a zinc-bearing solution containing chlorine, the steps of treating the zinc-bearing solution with a soluble silver compound and thereby .removing chlorine by precipitation to the desired extent from the solution prior to electrolysis for the purpose of reducing the corrosion of the electrodes during the electrolysis, and converting the precipitated silver chloride removed from the solution into the soluble silver compound which is then available for ythe removal of a further quantity of chlorine from zincbearing solutions.

7. In the recovery of zinc by electrolysis from a zinc-bearing solution containing chlorine, the steps of deilecting a portion of the zinc-bearing solution and removing chlorine therefrom to the desired extent and passing the substantially chlorine-free solution back into the remaining zinc-bearing solution, whereby the latter solution has its chlorine content reduced to the extent desired before electrolysis.

8. In the recovery of zinc by electrolysis from a zinc-bearing solution containin chlorine, the steps of deflecting a portion o the zinc-bearing solution and removing chlorine therefrom to the desired extent by precipitation with a soluble silver compound, and passing the chlorine depleted solution back into the zinc-bearing solution whereby the latter solution has its chlorine content reduced to the extent desired before electrolysis.

9. In the recovery of zinc by electrolysis ver chloride to metallic silver, and heating the metallic silver with the calculated amount of sulfuric acid to form silver sulfate which is then available for removing l chlorine from a further quantity of solution, the chlorine depleted solution being returned to the zinc-bearing solution whereby the chlorine content of the latter is reduced to the extent desired before electrolysis.

In testimony whereof we have signed our names to this specification in the presence of two subscribing Witnesses.

DAVID AVERY. ROYALE HILLMAN STEVENS.

Witnesses for David Avery: V. bG. ANDERsoN,

M. R. CULLEM Witnesses for Royale Hillman Stevens:

ALLAN MCINTYRE, V. S. CHAMBERS. 

